The invention relates to the field of optical communication, and in particular to high transmission waveguides and magnetooptical isolators having high Faraday rotation that can be integrated on Si and GaAs substrates.
Waveguides are typically made by deposition of the core material on a substrate of cladding material. Lithography is used to define the layout, of the waveguides, an etching step is used to pattern the waveguides, and finally a cladding is deposited on top. This typically leaves waveguides with minimal roughness on the top and bottom of the waveguide but substantial roughness on the sidewalls. There are currently two methods for reducing the roughness of silicon waveguides.
The first involves using an anisotropic etch which preferentially etches the surface to expose the slow etching {111} crystallographic planes. There are several detriments to choosing this smoothing technique. If the waveguide is crystalline, waveguides that direct light in different crystallographic directions will yield different cross-sectional shapes. This can affect polarization dependence, mode profile, and transmission of the waveguides. The final cross-sectional shape of the waveguide is usually trapezoidal or triangular depending on the original waveguide geometry. If the waveguide is polycrystalline, the effect of an anisotropic etch is unpredictable. Depending on the grain size, anisotropic etching can actually increase roughness. Not all aspects of anisotropic etching are detrimental though. Anisotropic etching has the possibility of smoothing a surface to near atomic smoothness (<4 A). The process is fast and requires no heat (the solution is often heated to 60–75 C to enhance reactivity): a plus if there is a tight thermal budget.
The second method for reducing silicon waveguide roughness is oxidation. Exposing the bare silicon waveguides to an oxygen rich environment promotes the growth of oxide. The high surface energy of the rough silicon surface is lowered by the oxidation as the crests of the roughness are etched fastest. The overall result is a smoothing effect as the oxidation occurs. There are several detriments with this method as well. First, typical silicon waveguide roughness requires several hours of high temperature oxidation to smooth the roughness to a level that allows for sufficient light transmission. This is dire for chip. designers who have tight thermal budgets and do not want diffusion to occur in other parts of the chip. Then again, the extent of cross-sectional shape alteration of the waveguides is not as drastic as in anisotropic etching. However, significant material is removed, and critical dimensional control is lost. Thus, oxidation smoothing is less sensitive to crystallographic direction, allowing for more freedom in photonic chip design.
The two methods described above have undesirable detriments. Thus, there is a need for a smoothing technique that allows for freedom in optical chip design, fits within thermal budgets, limits material loss and works with both poly and single crystal silicon.